1. Field
One embodiment of the present invention relates to a disk drive having bit-patterned media, and more particularly to a technique of measuring the phase shift of a write clock signal.
2. Description of the Related Art
In recent years, various techniques have been developed to achieve high-density data recording, as demand increase for large storage capacity in the field of disk drives, a representative example of which is the hard disk drive. Attention is now being paid to a bit-patterned media (BPM) system, which is a technique for achieving high-density data recording. The BPM system is a recording system that records data bits in association with magnetic dots or magnetic dot groups which are formed on a magnetic disk (i.e., recording medium) and which are magnetically independent of one another.
In the BPM system, the magnetic dots or magnetic dot groups (hereinafter, generally called “magnetic dots”) formed on the magnetic disk must be synchronous with the transition timing of the write current. (This synchronization will be referred to as “recording synchronization”.) If the recording synchronization is incomplete, the write current will undergo transition at, for example, the center of each magnetic dot provided on the magnetic disk. Consequently, the magnetization direction of each magnetic dot will become indefinite, resulting in a write error.
To prevent such write errors, it is necessary to generate a write clock signal of high precision that is synchronous with the magnetic dots formed on the disk. Methods of generating such a write clock signal have been proposed. In one of these methods, a preamble (region), in which a continuous pattern is recorded, is arranged at the head of each sector and detected in preparation of recording data, and a write clock signal is generated from the continuous pattern reproduced from the preamble.
Since the detection of the preamble is synchronous with the reproduced signal in the above-mentioned prior art, the timing of recording data in synchronism with the magnetic dots is indeed synchronous in terms of frequency with the magnetic dots. However, the write clock signal and the magnetic dots are not always synchronous in terms of phase. Consequently, precise recording synchronization cannot be accomplished. In the disk drive, the data reproduction circuit (read channel) and the data recording circuit (write channel) delay data signals differently, because they are circuits that operate independently. Further, since the write head and the read head are physically spaced apart from each other and are different in configuration, they differ in terms of delay time. Therefore, the write clock signal is hardly be correctly synchronized with the magnetic dots even if the detection of the preamble is synchronous with the reproduced signal.
In order to solve this problem, a data recording apparatus has been proposed, in which the write clock signal is delayed so that the error rate may become optimal, thereby to accomplish recording synchronization (see the above-identified document). In this data recording apparatus, however, the read channel must be optimized in order to determine the error rate. Moreover, the bits on the magnetic disk must be synchronized with the timing of recording data on the disk in order to optimize the read channel. In the data recording apparatus described above, an equalizer that performs waveform equalization on a read signal for a partial response is used to determine the error rate. However, the equalizer may not always be optimized in the apparatus. Therefore, it cannot be determined whether the error rate determined is optimal or not. As a result, the bits on the magnetic disk cannot be synchronized with the timing of recording data on the magnetic disk.
Further, the phase shift of the write clock signal must be measured at high accuracy and then adjusted with the phase shift measured, in order to generate a write clock signal that is synchronous with the bits formed on the magnetic disk.